Telephone Line Audio Interface ?

Edwards Finance > Phones

Q. What is the impedance of an analog voice grade line? How would I match the 8-Ohm impedance from my audio recorder to the telephone line? The red and green wires in a telephone cord are the audio lines. Which one is signal and which one ground? Could I just feed the audio directly into the line from the recorder using a capacitor to block the DC from getting into the recorder? If so, what value capacitor would I use to do this? What kind of load (how many Ohms) do I have to put across an analog phone line to maintain the connection?

A. A thousand ohms DC should sieze and hold the line. Simply use the RadioShack telephone line transformer that they sell for a few dollars. It is designed for 600 to 600 interfacing. Once the transformer is across the line, it will hold and maintain your connection, even if the phone is hung up. You don't need a cap. You can simply drive the 600-ohm winding on the recorder side with the 8-ohm speaker jack audio. The mismatch will not affect quality, only the amount of maximum audio level. An ideal equalizer would have a 600-ohm output to better match the line transformer. You can't make a silk purse from a sow's ear. (Old proverb) 1. Get your boss to buy you a laptop with a telephone modem and an audio card with a mike input. This puts the cost where the benefit is realized. 2. Record you story as a WAV file at whatever degree of Hi Fi you wish. 3. Attach the WAV file to an e-mail and send it to your station. 4. Let the station engineer figure out how to use it. He can. The Mike Sandman catalog has two different commercially made telephone line audio interfaces. One connects directly to the phone line. The other one connects to the handset cord of any telephone, and so can be used even with digital office/PBX phones and ISDN phones. http://www.sandman.com/ While these units are NOT sold for peanuts (I think they are around $100-200, new), you might find a used on up for auction on eBay. If you were to try to build your own, the parts would probably cost about $50, but you would have to design it, test it, and put it into some kind of convenient to use carrying case. One crucial requirement for any telephone interface is to couple the audio through a transformer, so that there aren't any ground loops. Decent audio transformers themselves are not real cheap.

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